1. Field of the Invention
The invention relates generally to circuit interrupters and, more particularly, to circuit breakers having magnetic drive devices to rapidly separate the contacts under high overcurrent conditions and produce current limiting action.
2. Description of the Prior Art
Current limiting circuit interrupters provide protection for electric power circuits by limiting the fault current downstream from the interrupter to a value less than the maximum fault current available from the source supplying power to circuit. It is known in the art to provide current limiting action by rapidly separating the contacts of the interrupter during fault conditions to quickly produce a high arc voltage across the contacts and oppose the flow of fault current. An example of a device operating in the manner is described in U.S. Pat. No. 3,991,391 (Class 335/16) issued Nov. 9, 1976 to John A. Wafer and assigned to the assignee of the present invention. Normal low-to-moderate overload current protection is provided by associated thermal and magnetic tripping circuit breakers, or an integral thermal and magnetic tripping capability may be provided.
One method of producing rapid contact separation employs a slotted magnetic drive device comprising an open-ended slot within which is disposed a movable contact arm. The contact arm is movable between a first position at the open end of the slot wherein the contacts permit current to flow therebetween, and a second position at the closed end of the slot which interrupts the flow of current through the contacts. During high overload current conditions, the current flow through the contact arm generates magnetic flux in the slotted magnetic drive device. The flux across the air gap at the open end of the slot interacts with current flowing in the contact arm to rapidly drive the contact arm from the open end of the slot to the closed end. The amount of force exerted on the contact arm (and, correspondingly, the degree of current limiting action) increases as the width of the slot decreases. However, for higher ratings, the slot width must be increased to raise the threshold of current limiting action above the upper setting of the magnetic trip function of the circuit protection equipment. If this is not done, an overload current at, or slightly above, the threshold level will cause the contacts to slightly separate but will not supply sufficient energy to activate the thermal or magnetic trip. Thus, current would continue to flow but at a level below the threshold. The contacts would then return into engagement with each other, at which time they would slightly separate once again, continuing in this manner to produce a "chattering" action. It would be desirable to provide a current limiting circuit interrupter which raises the threshold of current limiting action to prevent chattering but maintains a narrow slot width to produce effective current limiting action.